A typical data communications network includes multiple host computers that are linked together by a combination of data communications devices and transmission media. In general, the host computers communicate by packaging data using a standardized protocol or format such as a network packet or cell (hereinafter generally referred to as a packet), and exchanging the packaged data through the data communications devices and transmission media.
In the field of computer networking and other packet-switched telecommunication networks, the traffic engineering term quality of service (QoS) refers to control mechanisms for achieving a desired service quality. Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow. For example, a required bit rate, delay, jitter, packet dropping probability and/or bit error rate may be guaranteed. Quality of service guarantees are important if the network capacity is insufficient, especially for real-time streaming multimedia applications such as Voice over IP (VoIP) and IP-TV, since these often require fixed bit rate and are delay sensitive, and in networks where the capacity is a limited resource, for example in cellular data communication.
For example, a distinction may be drawn between packets carrying video data (i.e., video packets belonging to a video QoS class) and packets carrying general data (i.e., general data packets belonging to a general data QoS class such as Best Effort Service). In this arrangement, a data communications device processes video packets through a network differently than general data packets due to different link resource availability and resources being allocated differently based on the QoS class of the packets.
There are different types of QoS routing techniques. In one QoS routing technique (hereinafter called QoS class-prioritized routing), a data communications device internally prioritizes the processing of different QoS class packets in accordance with a pre-established QoS policy. For example, in accordance with one such QoS policy, a data communications device gives higher priority to video packets relative to general data packets. Accordingly, if the data communications device simultaneously receives a video packet and a general data packet (e.g., through multiple input ports), the QoS policy directs the device to process the video packet before the general data packet. As a result, in QoS class-prioritized routing, packet destinations (i.e., receiving host computers) generally perceive different responses, or Qualities of Service, for different QoS classes (e.g., faster video transmissions than general data transmissions).
In another QoS routing technique (hereinafter called load-directed routing), the data communications device generally directs packets through the network based on network load conditions. For example, suppose that the data communications device has multiple output ports to data paths that eventually lead to the same remote destination (i.e., the same receiving host computer). When the data communications device receives a video packet destined for that remote destination, the device transmits the video packet out the output port expected to lead to a low traffic area of the network. On the other hand, when the device receives a general data packet destined for the same remote destination, the device transmits the general data packet out a different output port expected to lead to a high traffic area of the network. With this arrangement, the video packet should travel through the network faster than the general data packet. Accordingly, the load-directed routing technique generally provides the same result as the QoS class-prioritized routing technique. That is, packet destinations (i.e., receiving host computers) generally perceive different Quality of Service or responses for different QoS classes (e.g., quicker video transmissions than general data transmissions).
A network or protocol that supports QoS may agree on a traffic contract with the application software and reserve capacity in the network nodes, for example during a session establishment phase. During the session it may monitor the achieved level of performance, for example the data rate and delay, and dynamically control scheduling priorities in the network nodes. It may release the reserved capacity during a tear down phase.
A best-effort network or service does not support quality of service. An alternative to complex QoS control mechanisms is to provide high quality communication over a best-effort network by over-provisioning the capacity so that it is sufficient for the expected peak traffic load.
Another processing technique involves splitting traffic among multiple output queues which are then scheduled in some fashion (often using an algorithm called Weighted Fair Queuing (WFQ), which provides a specified minimum fraction of the link bandwidth to each queue feeding the link). Diffserv services (described below) often uses WFQ between traffic classes, with each class having its own queue.
Random early detection (RED), also known as random early discard or random early drop is an active queue management technique. It is also a congestion avoidance algorithm. In the traditional tail drop algorithm, a router or other network component buffers as many packets as it can, and simply drops the ones it cannot buffer. If buffers are constantly full, the network is congested. Tail drop distributes buffer space unfairly among traffic flows. Tail drop can also lead to Transmission Control Protocol (TCP) global synchronization as all TCP connections “hold back” simultaneously, and then step forward simultaneously. Networks become under-utilized and flooded by turns. RED addresses these issues by monitoring the average queue size and drops (or marks when used in conjunction with ECN) packets based on statistical probabilities. If the buffer is almost empty, all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buffer is full, the probability has reached 1 and all incoming packets are dropped. RED is considered more fair than tail drop. The more a host transmits, the more likely it is that its packets are dropped. Early detection helps avoid global synchronization.
Per-flow QoS is also known as “intserv”, for the IETF working group that defined it. It typically used WFQ with a single queue for each end-user traffic flow (e.g. a single TCP connection, or all traffic between a specific pair of sites).
Another type of management technique is knows as Differentiated Services (diffserv). In diffserv, users pay for the desired bandwidth speed and allotment (collectively referred to as a service level agreement (SLA)) for a packet flow. A flow is marked at every router, the marking indicating the level of service the packet flow is receiving. For example, a green marking indicates that the packet flow is within the SLA, a yellow marking indicates that the flow is slightly over the paid for SLA (but not by a large amount), a red indicator means the packet flow is over its SLA. A flow may transition between different markings as it traverses a network. Yet another type of management technique is known as Multi-Level Priority Marking (MLPM). In MLPM the IP Precedence field of a packet is used to encode the value of packets in an encoded video stream, so that the least valuable packets are dropped first.